populate content assist from external source

I would suggest first looking at parametrised substitution actions - https://confluence.jetbrains.com/display/MPSD32/Editor+Actions
Cell menus in the editor definition may also be possibly used - https://confluence.jetbrains.com/display/MPSD32/Editor

Vaclav

Thanks Vaclav,

By using actions am able to add Custom items to the auto completion menu. But by default all the items get sorted by alphabetic order. can i change this behavior and sort the completion list depending upon my requirement. For example can i sort the list based on the usability i.e. most used actions appears at top of the list.

Lokesh

No, unfortunately, there is no such capability in the current version.

And is there any ability to make bold font? 'Cause right now custom items are not bold, while the items in menu generated by MPS for some concept automatically are.

No, MPS currently does not provide facilities to customise the font style of individual entries in the completion menu.