I'm wondering is it posible to create or modify Excel spreadsheets that are on SharePoint 2010 programaticaly and how to do it?
have a look at the OpenXML SDK. See link text for examples of what you can do.
You could also take a look at OfficeWriter, a product from SoftArtisans. It has the ability to create and modify Excel (and Word) documents programatically. OfficeWriter is much easier to work with than the OOXML SDK. It can also interact with SharePoint 2010 (and 2007). They have information about both OfficeWriter and it's SharePoint integration on their website at http://officewriter.softartisans.com/officewriter-434.aspx.
Full Disclosure - I work at SoftArtisans.