Certainly is something which is possible. People use custom fields for all sorts of things. I remember some time ago someone using an insert script for custom fields to indicate whether the asset was a physical asset or a VM so it was easier to spot.
The bigger question is how you want to get the information in Lansweeper. Static information isn't a big problem as you can simply enter it into Lansweeper. If you have different PM dates based on the type of device, it can still be easily done with an insert database script.
The real complexity comes when each device has it own specif PM date (which is the most likely scenario). Maybe by using warranty information the PM date can be dynamically calculated.
As you can see the implementation can vary.
