Sip walls are easy enough to draw in PlusSpec, personally I use the Grid tool
add the SIP size in the Grid spacing and left-click on the grid when drawing. This will section your wall sizes yet you will not be able to add a window on the join for obvious reasons. SIPS can be restrictive when it comes to customisation in construction so using the Grid will enable you to add the spacing according to where you want your windows and doors to go.
If you are looking to quantify SIPs I would use https://plusspec.com/plusdesignbuild/
and use recipes and associate the SIP name with the recipe. If you do not want to quantify the inner parts of the SIP simply exclude the items form the takeoff.