Flying in areas with famous mountains (St Helen, Everest, K2 ....) I would like to see the point on my ND.
So I should add a waypoint on the NAVDATA with certain coordinates and then insert this waypoint on the fix page.
How can I do?

The easy way to see mountains is to turn on the terrain mapping feature.  Sure, you could create a custom waypoint on a FIX page using lat/lon coordinates but now you are just playing around.

Ok, but if I want to add new waypoint in database with coordinates? Is it possible?

31 minutes ago, Attilio said:

Ok, but if I want to add new waypoint in database with coordinates? Is it possible?

Of course, this is discussed in a PMDG Knowledge Base article, in the FCOM in detail, and a quick reminder of the format required you can look at the format on the CDU for the GPS coordinates, e.g., N29 30.5 W090 45.2.  I am sorry, I assumed you already looked in the FCOM.

Yes I read FCOM 1 and 2.

I know how to insert waypoint with coordinates.

I want to insert waypoint in NAVDATA database out of my route.

1 hour ago, Attilio said:

I want to insert waypoint in NAVDATA database out of my route.

I am sorry, I don't understand.  It reads as if you want to add a waypoint from your route (which consists of waypoints based on navdata) into the navdata that is the source for your waypoints.

Every waypoint you put in your route, except for custom waypoints, are in navdata; otherwise, you would get a 'not found' message.  Possibly you want to enter custom waypoints into navdata?  All global waypoints are in  the navdata file wpNavFix.txt.  This file is a text file that you may edit with any text editor but be carefull, the columns are important and all white space are space characters, no tabs.  Everytime you update navdata you will have to reedit your changes to navdata.

Honest, it is quicker to just set the FO ND to terrain mode to see those mountains.

For example in a320 there is the possibility to create additional waypoints that end up in a temporary database. Is there any chance on the 737?

4 hours ago, Attilio said:

For example in a320 there is the possibility to create additional waypoints that end up in a temporary database. Is there any chance on the 737?

I haven't seen that in the FCOM.. probably not.  Why would one find such a thing useful?  I keep chasing this but still don't understand the problem you are trying to solve.

You can add a custom waypoint at the end of your route, separated by a discontinuity. That way the waypoint will appear in the ND but is not part of the active route.

17 minutes ago, kevinh said:

You can add a custom waypoint at the end of your route, separated by a discontinuity. That way the waypoint will appear in the ND but is not part of the active route.

This is exactly how I learned to plot the ETP in the NGX

34 minutes ago, downscc said:

This is exactly how I learned to plot the ETP in the NGX

That’s where I got it from Dan. Matt Cee (spin737) described the procedure in this very forum. 

I am assuming that he wants to add user defined waypoints to the database, so that he does not need to keep adding the FIX coordinates of new waypoints every time that he prepares a flightplan in the FMC.

I don’t think you can do that in the NGX. You could manually add the fix to your navdata file, it’s just a text file, but you’d have to do it after every AIRAC update. 

https://www.pprune.org/tech-log/521615-custom-waypoints-b777-vs-b737-fmc.html

I did some research. The 737 has a temporary database. In navdata just write a waypoint name not already present in the database and then enter the coordinates.
All temporary points will be listed on the summary page.
Apparently this is not simulated