Yes it is possible.
From the help file
/sline [-a|r] <#channel> <N|nick>
Selects or deselects lines in a channel nickname listbox. It can select either the Nth nickname in a listbox, or a specified nickname.
If you do not specify any switches, any existing selections in the listbox are cleared. If you specify the -a switch then the specified is selected without affecting the selection states of other lines. If you specify the -r switch then the specified item is deselected.
Thanks for letting me know. I was looking in the Custom Windows section of the help file and overlooked the -a switch.
