We recently added several Dell E310dw printers to the network and LanSweeper consistently picked them up as a switch. I tried adding a few custom OID's for it to identify the printer, but it seems when it hits the values in "1.3.6.1.4.1.674.10898.2" it identifies it as a switch. Doing a quick search of the built in OIDs, I see "1.3.6.1.4.1.674.10895" all appear to be PowerConnect switches so I'm assuming it hits that and assumes that's what it is and stops looking.
Based on SNMPWalk data, I added "1.3.6.1.2.1.1.1.0" with "Dell Printer E310dw" and "1.3.6.1.4.1.2435.2.4.3.99.3.1.6.1.2.1" with "Printer E310dw", but neither had an affect on what it automatically identified as, still a switch. It does pull pages printed and printer status, but it doesn't get toner or anything, classifies as a swtich, and shows the network interface list like a switch does.