From what I recall of a previous discussion along these lines, the short answer is no, if you have already submitted records you cannot retrospectively associate them with an activity manually. However, there are various different settings linked to activities. I have an activity for my local patch and the settings allow all records that meet my criteria (i.e. within a set boundary area) to show up, regardless of whether the person submitting the record has used 'my' recording form. It might be that your local natural history society has a similar setting, or could change their settings along those lines in order to capture the maximum number of local records.