It is possible to link other records to an activity by adding the record's ID into the activity description with a preset prefix.

For example add  "CRMID:123" to an activity description to link the activity to the record with ID 123. 

This will also update next and last actions for any new linked records via this method

The prefix to use is controlled by the following setting:

"Prefix for tag in description used to link activities to existing records by matching by ID"
and defaults to "CRMID:". 

This can be used from the OpenCRM web app  itself, or, for example if creating an activity from Jotform, or some other external source.