At current, TB mechanic works like this.
Players are in TB encounter, and have their set battle sequence.
A NEW player enters, and they must wait till the end of the existing round, to be inserted in the sequence.
SUGGESTION
-As soon as you enter a TB fight, you are entered into the sequence after the existing players turn. So as long as your sequence is higher than the next players turn, you can basically spawn and shoot/move.
This would drastically change the nature of TB traps/battles, and the strategies about the employment of sequence.
If there are concerns that this could cause some issues, apply a roll or minor penalty (You enter at -5 sequence, unless you have X perk or whatever).