I'll try to answer based on the example you gave.
Let's say you have two DynamoDB tables, one in us-east-1 and another in us-west-1. As per DynamoDB's terminology, these two tables are replica tables. Now, you want to add another table in, say, eu-west-1.
The answer is, you can, but only if the following constraints are met according to this documentation:
- The table must have the same partition key as all of the other replicas.
- The table must have the same name as all of the other replicas.
- The table must have DynamoDB Streams enabled, with the stream containing both the new and the old images of the item.
- None of the replica tables in the global table can contain any data.
In other words, your tables in us-east-1 and us-west-1 must be empty. If they aren't, you won't be able to add the one in eu-west-1 to the global table.