Yes.
Your SoftRAID and MacDrive license enables you to add more seats at anytime.
SoftRAID
To add a seat to a SoftRAID license, you will need to login to your account and go to the “Subscriptions” section. Click on the active subscription and you will see “Subscription totals”.
Here you can click the “Change subscription” button and adjust the seat count.
MacDrive
To add a seat to a MacDrive license, visit the License management page, enter your serial number, and there is an option to add seats.