you can do it, of cause.
Smart redundancy in previouse versions required identical model and license size for both ZD. So up to 9.8 it works this way:
When you'll buy and register RDY support for ZD3100, you'll get for it free additional 150 licences to make size identical, than you can create redundant pair. If you need additional licences, than you have to add them to main ZD. This still works when you upgrade to 9.9 or 9.10.
Question is about licences on backup ZD -- you have 100 and in version 9.9 you can reuse it probably.
From 9.9 release you can pool licenses from both ZD. On ZD1200 it works this way. Procedure to register for redundancy ZD3000 is different, but I see that ZD3000 in pair now shows double license q-ty in dashboard after upgarde to 9.9. I suppose you must be able to reuse all your 100 licences on backup ZD, but I haven't tested it actually yet. You have to check with Ruckus support, they obviously can help with this.