Like the old Defense Stations? Paradox removed that feature from the construction ships, though it may be possible to add that again, I don't know how. What do you specifically want to do?

same as the megastructure but at less cost more time and with the added threat of enemies trying to blow it up as a remover so it feels more impactful

Ohh, I understand. Yes, megastructures can't be attacked by fleets, so you want to add that capability by making the emitters and receptors normal space stations. Like I said, sadly I don't know how to add that ability back.

exactly it gives the option for a desperate last minute battle as you try to open or close a lane to either cut the enemy of or get reinforcements to your ailing fleet fleet

Can you make a alternative version by converting the megastrucures into starbase buildings?

Probably. But I won't, because:
1) It would require doing monthly checks for the events, which can impact performance.
2) It wouldn't change anything since I would still put the same tech requirements and the same costs.
3) I don't want to.