Yes, you can do this upgrade and depending on how many batteries you're adding, you can change the converter over to Lithium by changing the position of the Converter Output Mode via a tiny microswitch to the Li position.
On page 2 of that manual, you'll see a diagram showing where the Converter Output Mode switch is.
And here's what the manual says about it:
OPTIONAL OUTPUT MODE SWITCH: The optional OUTPUT MODE SWITCH sets the converter output to either a constant 14.6VDC with the Charge Wizard® disabled (switch in ‘LI’ position) or a nominal 13.6VDC with full Charge Wizard® function (switch in ‘LA’ position). ‘LI’ mode is intended for use with lithium batteries with a BMS requiring a constant converter output. ‘LA’ mode is intended for use with lead/acid batteries where the Charge Wizard® will optimize battery charging.