It shouldn't take any more time to add a row than it takes to enter data into a field of that row.
But a script can create a set of records in your portal table. There is more than one way such a script can do that depending on details not available in your original post.
Option 1, If you have a set of records in a table that can serve as a "template" for the set of related records, a script can use Import Recors to import this data into a portal table and then Replace Field Contents can be used to update the foreign key fields with a value that matches them to the current record in the parent table.
Option 2, A script can copy the value of the current parent record's primary key to a variable. It can then switch to a layout based on the portal's table and use a loop to create a set of new records, using Set field to set each new record's foreign key field to the value of this variable in order to link them back to the current parent record.
Option 3, Using the same methods as 2, a script can set up a portal record and then use Duplicate record inside the loop to create the new related records.