Break of journey is generally allowed on walk-up tickets. Except when it isn't. The whys and wherefores of those flows that disallow a
BoJ▸ are numerous and it'd be a treatise to explain them all.
However, what you are proposing to do is absolutely fine. On an Off Peak Day Single from Axminster to Tiverton Parkway you are free to break your journey at Exeter Central. I'm pretty sure I've done what you are proposing more than once - completing the circle with an add on ticket and breaking my journey on said ticket. Whilst the ticket might not operate the barrier at Central, a quick explanation to the gateline staffer - "I'm breaking my journey here" - such be all the explaining needed. No need to complicate things by mentioning the
FOSS▸ Rover.
Just today I broke my journey with a walk up Off Peak ticket at Basingstoke. Ticket didn't work the barrier. I said exactly as above. A cursory glance at my ticket by the gateline staffer and I was let through.
EDIT: Or what rogerw said.