When you say 'with users in K2', what do you mean?  K2 gets its users from a security provider--out of the box ActiveDirectory or SQL tables.  But there are other choices.  You could maintain lists of authorized approvers in many different ways.

Let's take an example.  You could create a SharePoint list of authorized users for a particular activity.  This SharePoint list could be added as an instance of the SharePoint ServiceObject and then you could connect to it with a SmartObject.  Your SmartObject could then be added as a secondary data source to the InfoPath form connected to your drop down list.  For the desitnation in the next step you could select the XML node where the value of the drop down list is stored and use it as a destination.  All of this can be done with no code.  Plus, you have an administrative screen provided by SharePoint where authorized users can edit the list of approvers.

If you don't want to store the users in a SharePoint list, you could easily use SmartObjects to get them from somewhere else.  There is even an ActiveDirectory service that you can connect SmartObjects to pull data from there.

The good news is you have lots of options!

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