I would imagine they'll eventually have a way for you to interact with the retainers from other characters on your account, but for now, there are a few ways to transfer items/gil. First, if you have people you trust in your LS, then simply trade them what you want to transfer. Second, if you want to transfer gil and don't mind losing a %, you can put a basic item in your retainer's bazaar for a large amount. You can do the same with items, of course, but be careful to put them in the proper ward so you don't lose more gil than you need to.
Probably the best option for now is finding people you trust so you don't lose gil.