create workflow using aspx form

What are you looking to do with the aspx page?  you would have to use the K2 api to interact with the process.  This would involve establishing a connection to the K2 server then kicking off the process and or opening a worklist item and actioning it.  There are some good code exxamples here

http://help.k2.com/en/k2devref.aspx

We do this exclusively in our shop.  All of our processes use .NET web apps as their front end.

The K2 API is fairly complete for interacting with and creating process instances, among other things.  In the link posted above, start at Processes > Accessing, or see here for an example.