I don't know of one, but are you sure that's a good idea?
What you are asking for is a "swap file". The phone won't be able to use the card as working memory (and you wouldn't want it to - it's much slower than RAM) but would be "paging" stuff between the card and real RAM, i.e. making a copy of stuff that would otherwise be cleared from RAM on the card and copying it back when needed again, probably having to swap space with stuff that was currently in RAM. I'm sure you'd need root to set this up, probably a special partition on the card too, but as you can probably guess from the description it won't result in a faster device.
Of course I'm assuming you are using the word RAM correctly. If you meant expanding the internal storage to allow more apps to be installed look at Link2SD (requires root).