I had set up an F-Ssure account using a free month trial subscription. Subsequently I bought an annual license when a special offer came along. Is there any way of attaching this subsription to my previous F-secure account?
Sorry for my reply. I'm only F-Secure user (their home solutions).
Thus, it is only unofficial feelings and suggestion.
In general, I think that it should be possible to check "My F-Secure Account" portal.
For example, login into your "My F-Secure Account" portal -> switch tab to "Order/Payment" (or if there is an option to 'add' subscription code from first) and check if it is possible to use something like 'add subscription code' (or so).
With your certain situation -> I think that can be limitations.
For example, some of F-Secure official 'special promos' with restriction for 'only fresh users' (so, need to create an account from first with certain promo-code or subscription-code; or with specific place).
That is fresh account which pinned to special subscription.
I think that if it is your situation - maybe possible to try contact their direct F-Secure Support Channels (chat as example) for clarification: https://www.f-secure.com/en/web/home_global/contact-support
Did you want to attach subscription to previous F-Secure Account for avoid use of multiple mail-addresses? Or for avoid multiple F-Secure Accounts?
If you log in to your SAFE Portal, assuming you have previously registered, you should see some options similar to these, so you should be able to enter the new subscription code.