recon-ng already supports import of external data (by newlines) with the SOURCE variable. SOURCE should be available via most plugins, including the domain ones you are interested in.
Check the wiki -- https://bitbucket.org/LaNMaSteR53/recon-ng/wiki/Usage%20Guide -- which includes information on using the SOURCE variables for data from the local filesystem.
There are also the
import/list modules in recon-ng which will push your data into the recon-ng database.
The method for creating a workspace and using
add domains to add domains to that workspace's database can be seen in this blog post -- https://www.stevencampbell.info/2016/10/passive-information-gathering/ -- after which you can see the domains along with IP-address data by way of
show hosts. This appears to be the way that you are adding domains, which is one way of doing this.
Once a domain plugin is run, or if a workspace is created and domains are added -- as long as you see your domain via
show hosts, then the recon-ng database (usually best performed per workspace) can be modified just like any sqlite3 database. For more information on where recon-ng places these sqlite3 databases on your local filesystem as well as other tips and tricks, see this blog post --https://www.codemetrix.net/practical-osint-recon-ng/