Clint, you can use whatever fonts you want in Marketo assets, including setting purchased fonts as the default.
But showing fonts into the Rich Text Editor dropdown is a different matter, and it's only possible if you have a very skilled developer (not officially supported).
And also, pay attention to what you are doing in emails... It's fully safe in landing pages, but some email clients (our beloved outlook) may not display correctly certain font and, as a result, your emails might look very weird, so test it before sending.
If you are looking for custom font support we have that with our Pro email editor in Knak. It's fully integrated with Marketo so you get the best of both worlds.