Can I connect the WT1800/WT1800E Precision Power Analyzer to the SMARTDAC+ GA10? It’s not listed in the supported devices in the manual.

Yes. Connect the WT1800 via GateWT for GA10 (free). If you are able to change the WT1800E command type, change it to WT1800 compatible, and then connect via GateWT for GA10. If you can't change the command type, connect using Modbus. See “Registering Modbus Communication Devices" in the user’s manual.