I noticed that if I level off during a climb for whatever reason after a takeoff, let's assume I level at 5000ft, than if I descent to 4000ft and after that I try to climb to cruise altitude, the fmc will give me only the DES page in PERF, is there a way to go back to CLB and CRZ pages?

Just a guess since it has been a while since I flew the MD11, on other types if you Re enter the cruise altitude in the INIT page it will force it back to CLB

Yes, that is correct. The plane goes into descend mode. One way to fix it is by going in the perf menu and press previous page. That should show the crz page. From there you can re-enter the crz altitude. 

Oops, got mixed up with the NGX. Rob is absolutely right. The perf pages disappear so to make them reappear a new crz altitude is required in the init page.