Guild Wars 2

I’m thinking of making a set of defensive exotic armors,
so that I can use both my offensive and defensive armor set.

But making another set of armor is costly,
so…I got to thinking,
‘is it possible to just overwrite desirable insignia on the already existing crafted armor?’

I wonder if such mystic forge recipe exists.
Or if not, if such thing could be implemented in the future.

No, there is no way to turn gear with one stat distribution into gear with a different stat distribution, without first obtaining a set of gear with the desired stat distribution.

It is not possible to replace the insignia/inscription of existing gear.

The mystic forge already has something a little like what you describe, but not what you want:
If you insert 4 same type items (e..g. 4 sets of boots) of the same quality level, you will get a random item of the same type, and of the same or better quality level and level requirement.

You could consider posting your suggestion on the Suggestions Forum.