Hi @Miranda282576069jcq,
Thanks for the question.
A simple way of solving this issue would be to use an anchor point. You could paint your veins inside a dedicated mask, add an anchor point to this mask and link it to your .SBSAR.
In the Designer's graph, you'll have to add an input that will contain your mask. Inside Substance 3D Painter, when you'll chose the input content of your .SBSAR, select the anchor point previously made.

Tell me if you need anymore guidance.