Hi commu,

I wanted to take on this Senior C. Galumalemana on a temporary basis via Randstad to make up for an injury shortage I had in my youth team.

However, I see that he's protected by the labour code and that I can't sack him because he's due to retire soon... Given that we can no longer convert newly-arrived players into staff, I'm therefore condemned to wait until he retires?

Yes.
You can't sell or release players in their final season.