Marketo doesn't have a lot of calculation tools. Adding scores is impossible with it. That being said, most CRMs have this, and if you're data is also syncing to a CRM you can have you CRM do the lifting if you want. It could get a little messy, but create a field in your CRM that does whatever you want it to (i.e. Score = field1 * 5 + field2 * 3 + filed3 * 2) and when data is submitted have marketo sync the lead to your CRM, pause for say 5 seconds, then have it re-sync the lead to get the calculated result.
Messy, and without knowing more your specific application I can't say if it would work for sure, but at least its an avenue to explore.