I am a Norwegian. Norway is a part of EFTA, but not the EU.
I am currently pro EU, but also fairly pro socialism, or at the very least pro social democracy.
I want Norway to have the security guarantees the EU offers, I want the same currency, I want Norway to be able to participate in changing EU, not just accepting something Brussels have decided without our participation (EFTA). And several other reasons.
We can currently veto implementing some decisions, but not too many.
If Norway joins the EU will it make it harder for Norway to transition to socialism/maintain social democracy?
No, absolutely not, EU was created as a neoliberal program. Remember what they done to Greece at the mere shadow of threat Greece not even becoming socialist but merely trying to make a tiny crack in neolib austerity dogma.