Yes, you could bulk load what will be Primary DC1 as a non-region single DC cluster, and then convert it into a multi-region setup. You could also just write a regions configuration that is one region, and only one DC, which is Primary DC1. The two approaches will be roughly equivalent.
Meng answered most of this, so I won’t repeat, but this is a trick that you can use to save yourself some WAN bandwidth also. Data distribution isn’t smart enough yet to only do one copy across the database for replicating a primary triple replicated cluster into a secondary triple replicated cluster. If you add the secondary DC as a single-replicated DC, and then once a full copy of the database is in the secondary, convert it to triple replicated, then you’ll save yourself the WAN cost of the other two copies.