You have some possibilities:
1) Actually Excel (and practically all Office application) support Add-ins. Thus you could make a legacy excel add-in. There are several tutorials out there, but this is not the simplest one.
2) An other interesting approach: http://exceldna.codeplex.com/
3) You can call managed code from excel vba, but it is not the wisest one, since vba runs in a rather undeterministic way. Start here: http://richnewman.wordpress.com/2007/04/15/a-beginner%E2%80%99s-guide-to-calling-a-net-library-from-excel/
4) Interop Forms Toolkit
] might be also an alternative to consider.