Can I "call" (start) another input script from the current one?

Yes. You can start a brand new input script from the current one. However, keep in mind that GuiXT will not return control to the original input script after the new one has finished.

Example: let's say you have two input script A.txt and B.txt
Contents of A.txt

Screen ABC
Enter "/nmm01" "process=B.txt"

//Note: The following portion of the script will never get executed if Screen ABC is processed

Screen PQR
Enter "xyz"

When GuiXT executes A.txt and it processes screen ABC, it will execute function mm01 and start with a new input script called B.txt. The control does not return to A.txt again