The place fro Feedback and "Wishes": http://feedback.photoshop.com/photoshop_family/products/photoshop_family_photoshop_lightro om
You could probably copy and modify the current lens profile. Here's how to find the files:Lens Correction Profiles « Julieanne Kost's Blog
... corrected link ...
I had already found the files but, after searching through Adobe Lens Profile Creator or Adobe Lens Profile downloader, I have not found the way to create a new lens profile correction starting from an existing one.
At your own risk ...
Find this file on you computer: PENTAX (smc PENTAX-DA 21mm F3.2 AL Limited).lcp
Copy and rename the file as appropriate, i.e. PENTAX (HD PENTAX-DA 21mm F3.2 Limited).lcp
Open the file and rework the :LensID, the :Lens, the :LensPrettyName, the :ProfileName in all places they appear appropriately.
Restart LR and give it a try.