Can I add solar panels to these UPS systems.

Technically yes, but it is not recommended. To effectively make use of power generated by solar panels, the energy must be stored in the batteries during the day, and used at times when the sun is not shining. So the batteries need to be “cycled” each day. To make these UPS systems cost-effective, batteries are used that have up to 300 cycles. For solar application, one would typically use more expensive batteries with one thousand cycles or more at least. Lithium-Ion batteries can have up to 5000 cycles.