i wouldn't recommend it as plastering sand is kinda rinsed and does not produce a "fatty" mix. maybe if you mixed a little hydrated lime into the mix then i guess it will be more suitable.
the most important thing is, that it matches the surrounding pointing.
It will slide of your trowel and end up down the wall or on the floor, be crap to work with but above all it will dry a grey colour which will be completely different to normal mortar.
Plus, it will lack the required flexibility and be too strong, so will end up cracking once water gets in and freezes and lead to failing prematurely.
For the sake of 99p, I would get some building sand
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