1. I have company loaded already.
2. I want to import existing company but custom property I want to use as a unique identifier not the Domain.
Hi @tanmay
You can’t change the deduplication property to be a custom property.
When reimporting existing company records you need to use the name or domain properties, or you can first export the companies with their unique identifiers.
More info on this last point: Deduplicate contacts, companies, deals, tickets, or product by object ID
Hope this helps.
Why isn’t it possible to use custom UID for dedublication? This is a limitation and forces me to find a workaround when I want to update existing contacts manually coming from another source 
I agree with this - it is quite a big limitation at the moment.
We have companies (restaurant chains) that have the same names and domains, even though they are with different addresses. But we do have UID that differentiates each individual record - how can we then import these records?
Hi @MYonova
Thank you for reaching out.
This is not possible at this stage within HubSpot, but I do understand how it could be useful for you - I’d recommend upvoting this Idea in our Ideas Forum regarding this. You can also subscribe to the idea (Options > Subscribe) to get notified if the status changes.
Thank you
Best
Tiphaine