I must preempt this by saying that I haven't actually tried this but...
Adding references to the infragistics modules may help you recognize the objects. Try Tools -> References (or maybe .NET References). See if the infragistics stuff is listed there.
I have had experience with Mercury QuickTestPro and Infragistics. Infragistics sells a product called "TestAdvantage". It is a supplement to the .NET add-in for QTP. It is supposed to help you recognize Infragistics objects, but we still had problems.
I would be very interested to know if how it works out for you in TestPartner.
In TP 5.3 there is no object level recogintion for 3rd party .net controls. This is the same case as QTP before purchasing the $4,000 TestAdvantage add-in from Infragistics. However, TP has the ablity to automate these controls via their native methods and properties.
For more info, look at the TP help Help~Index and the topic .NET > Extending .NET Object. One of the examples used in the help is from the Infragistics Ultra Grid. There are also numermous posts on this forum that you can find using the SEARCH.