This is for PC players, or maybe even console players if you know how to.
Step 1. Open the Coalesced.bin within the files with this utility here.
Step 2. Find this code and highlight it:
MemberValidCID=22, MemberAvailablePlotLabel=IsSelectableProthean,
Step 3. Copy this code and replace the other code with it:
MemberValidCID=, MemberAvailablePlotLabel=,
There you go, you have a new squad member "Javik the Prothean". He'll be fully accessible.