Will my existing merchant account work?

If you already access your merchant account through one of the supported gateways, it should work with Merchant for Android.

If you have an existing merchant account, you maybe able to access it by signing up with one of the supported payment gateways, as most "retail" merchant accounts go through one of several clearing houses that are also accessible via these gateways. You'll have to pay for the gateway account only, since you already have a merchant account.