This is an old post, but I have a similar need. My Solarwinds NPM polling was not retrieving the Service Tag as the serial number, but instead a long different value.
What I did was this:
I had a spreadsheet with all the Service Tags of my DELL switches. Just 2 columns - Switch Name and Service Tag
I created a custom property named DELL_Service_Tag and imported the value from the spreadsheet.
Matched the column Switch Name to Node Name and imported Column Service Tag to DELL_Service_Tag custom property.
The Custom Property displays correctly
I only have ~ 30 DELL nodes, so this was a relatively easy process. If I had > 100 then it may have been painful.
I wish Solarwinds would be able to read DELL Service Tags such that EoL\EoS dates can be automatically populated.