The 7290 I had many many years ago was out long before MicroSD Cards existed. This is a really old BB. I believe you're out of luck. By the way, I still have it and I do put in my SIM card and play with it about once a year for a few minutes. Still works good.
And very little internal memory if I remember correctly.
Actually it had a whopping 32 MB of memory, doubling the amount that past models had, and causing quite a stir in the BB community at the time. Words like "revolutionary" were bandied about, and folks dreamed of as yet unheard of capabilities. Heady times indeed!