I might be stating the obvious here so forgive me, but you can use ASP.net forms. See the documentation section - K2 Tutorials > Building Forms
in the K2 Designer for Visual Studio > Building Forms in the K2 Designer for
Visual Studio - Overview

I have a feeling though that since you reference InfoPath you would like a "no code" approach. If this is the case I don't know of another alternative.

Hi.
I used to build my forms in ASP.NET but I'm currently a big fan of InfoPath forms for the speed of delivery we can achieve.
It certainly has limitations that need workarounds and tricks to address,
but it seems InfoPath + SharePoint 2010 has got a lot of optimizations, and I am planning to upgrade soon.

However, I am sure that sometimes the best solution for complex problems is to get your hands dirty with some ASP.NET code.

regards.

You want something like this

http://www.k2.com/en/displaycontent.aspx?id=1021

i forgot to add this http://www.inobitsinc.com/Documents%5CInobits_-_iTechnology_-_Forms_Accelerator_Overview.pdf

Wow,
this really looks promising.
I'm interrested to know if you are using this solution and how it compares to infopath.

Regards.

Unfortunately i didnt use it; i just want to help.