I usually create my flight plans with SID, STARS, transitions and arrivals, almost all with altitude restraints. I love the VNAV capabilities of the Nxi .Then, the flightplan goes imported into FS, but G1000NXI recognizes every single fix except for altitudes. Is there a workaround for that or is it just a matter or entering the route via G1000 only?

I have the same problem. LNM doesn't export these values into the "MSFS 2020 PLN"-file.  If you open the *.PLN with a text editor you'll see your cruising altitude:

Quote

<CruisingAlt>6000</CruisingAlt>

But when it comes to waypoints you'll see the GPS position without altitude info and ┬░ looks strange, maybe a Unicode problem on my side with the text editor.

Quote

        <ATCWaypoint id="LABUL">
            <ATCWaypointType>Intersection</ATCWaypointType>
            <WorldPosition>N48┬░ 17' 57.00",W1┬░ 53' 22.00",+000000.00</WorldPosition>
            <DepartureFP>LABU4T</DepartureFP>
            <RunwayNumberFP>10</RunwayNumberFP>
            <ICAO>
                <ICAOIdent>LABUL</ICAOIdent>
                <ICAORegion>LF</ICAORegion>
            </ICAO>

There is an option to export "P3D or FSX PLN" and that PLN seems to look a bit different but better:

Quote

        <ATCWaypoint id="LABUL">
            <ATCWaypointType>Intersection</ATCWaypointType>
            <WorldPosition>N48° 17' 57.00",W1° 53' 22.00",+006000.00</WorldPosition>
            <ICAO>
                <ICAORegion>LF</ICAORegion>
                <ICAOIdent>LABUL</ICAOIdent>
            </ICAO>

Maybe we should try using the FSX PLN, that could be the workaround. I'll check that later. 

3 hours ago, Aristoteles said:

I usually create my flight plans with SID, STARS, transitions and arrivals, almost all with altitude restraints. I love the VNAV capabilities of the Nxi .Then, the flightplan goes imported into FS, but G1000NXI recognizes every single fix except for altitudes. Is there a workaround for that or is it just a matter or entering the route via G1000 only?

I suggest you check actual Garmin NXi and support documentation if the NXi modeling for MSFS is true to form.  I know for example that Garmin applies quite a number of rules/filters during import of external created flight plans in earlier GNS and GTN GPS models. That is more than obvious when using RXP GPS simulations integrated with the actual Garmin Trainers.  Protective logic is clearly in place in the trainers.

There is likely nothing that prevents attempts to import the same flight plan again and again over a range of months or even years, and variables like altitude restraints can change over time.  It is likely a liability consideration.  Again, this would apply more and more if the MSFS NXi is closely modeled on Garmin logic.