The first question must be, do you have the right to use these fonts online? Online use is often licensed separately. If you do, you need them in a suitable format. woff2 is probably the best choice as it is widely supported and smaller than the original woff. There are online converters which can make woff2 versions of your fonts. Now, whether you can use these in Foundry without an extra stack, I don’t know. Looking at the Foundry documentation, it doesn’t look as though you can – you only seem to have a choice of Google fonts or websafe fonts. I suggest you ask Elixir. You might try Fontamental from stacks4stacks.com which is only £13.88 and you can currently get it for 40% off with their Black Friday deal. By the way, there are many great free fonts at fontsquirrel.com.
As @Fuellemann points out, Foundry’s Typeface stack allows you to self-host your own fonts and tie them into the Typeface controls of Foundry. The documentation page for Typeface covers what the controls do, and there’s a tutorial video on that same page that walks you through using self-hosted fonts: