Hi,
There is a "parent" and "child" structure for custom fields as well.
You can group the specific custom fields and put them under 1 custom field.
- Let's say, you have 10 custom fields, and you can group them and put them under 1 parent custom field.
Through the products, when you add that 1 parent custom field(parent), all other 10 custom fields(child) will be added automatically.
If you add an extra child custom field to the parent, then this child should be reflected all the products (that has the parent child custom field).
BUT!! you would have done grouping and setup everything at the first place.
This is the only trick I know. Considering you have already added the custom fields to products individually and did the setup... i don't think there is an easy way for you. You need to add that extra custom field one-by one manually.
Through PHPMyAdmin by SQL Queries you can do it easily as well, but it requires expert knowledge on SQL. I won't recommend you to attempt, you may mess up everything.
Regards,
Support