If it supports the s-pen; it would be built in. They could not make the spen optional as the s-pen itself is actually the inexpensive part. It's the Wacom digitiser which would have to be built into every handset (just on the off chance someone want to get an s-pen) and that would represent a very expensive additional manufacturing cost overall that could only makes sense if they do ship with the spen. Which if it did, I'm sure Samsung would design/build in a holster into the device itself.
Honestly though ...
I don't think we will see s-pen on the S8. For the reasons of the extra manufacturing cost and when consumer don't expect an spen with the S range anyway and therefore 'returned gain fiscally' for Samsung adding that additional cost to the manufacturing.
Samsung need to maximise profit big time this year, and I can not for-see them doing that by adding an additional costs to a product where it wouldn't be needed / or the return on that cost would validate it.