Not quite sure what you mean. Are you proposing to add a polyA tail to to a linear DNA template (somehow), and then run an in vitro transcription?

This seems...unnecessary: loads of expression vectors incorporate a polyA sequence right after your GOI insert region, specifically so they can be used in in vitro transcription.

So: yes, adding a polyA tail to your template is fine, but for your purposes I suspect using a PCR with polyA-incorporating primers might be easier? Or just clone your insert into a plasmid with a polyA already there: one advantage of this latter approach is it allows more space for upstream binding. T7 can transcribe from fairly short, compact templates, but in my experience it seems to work better when it has a fair bit of upstream sequence to stick to first.

Hi John, yes the DNA template i am going tonuse for in vitro transcription will have the poly(A) tail. I wonder whether that will affect the transcription like a stop signal. Thanks

No. Transcription doesn't use stop signals like translation.

Translation is simple: UGA, UAA, UAG = stop.

Transcriptional termination is a fairly unwieldy process with multiple different methods employed in both eukaryotes and prokaryotes, often involving stem loops and termination factors and a lot of mess, and of course in euks this is then followed by cleavage at the polyA site and addition of a polyA tail by a polyadenylase.

If you just...add the polyA yourself, you short-cut all of this, and your transcription reaction is terminated by the very straightforward mechanism of "T7 reaches the end, and then falls off".

So no: it'll be fine. Just make sure the polyA is on the coding strand.