I have a portable 500GB external drive and would like to use the extra space for disk images.
Is there a way to create a Free Space Profile/Partition?
Thanks

No. The TechTool Protogo installer erases the entire destination device, so even though you could create a Protogo device on the 500 GB drive, and then use Disk Utility to split the volume to create a new one for the disk images, the next time you create a Protogo device on the drive, the partition with the disk images would be erased.

What if your needs don't change, so your device doesn't need to be revised, it would work wouldn't it?

I see no reason why you could not use Disk Utility to split the Protogo volume on a Protogo device to create a free volume, as long as you had at least 32 GB of free, contiguous (all in one piece) space at the "end" of the Protogo volume.