In the Starfinder Operation Manual the new feat Shield Proficiency says:
"Shield Proficiency (Combat)
You are trained in the use of shields.
Benefit: You gain proficiency with shields. See page 124 for
more on shields.
Special: Mechanics with an exocortex, solarians, soldiers,
and vanguards gain proficiency with shields automatically
at 1st level."
so those classes get that proficiency retroactive.
Is this book added?
I can’t retroactively add a feat that doesn’t exist in the sources.
OP is requesting a non-existent feat (From a non-coded book) be added to classes retroactively. Per PCGen rules, the book has to exist in the sources to apply.