Is there a way to manually add entries into a calculation field? I have a list of vehicles and when I select from a drop down menu, another calc field opens with the specific details of that vehicle parameters. If I use a vehicle not in the drop down list, I select "Other" and enter the ID manually, but when I try to enter that vehicle parameters in the calc field, it will not let me. How can I override that? The parameters themselves are used to make a summary so I need the information on the "other" vehicle to be included.

You can not modify a calc field. You can use a field with calculated result and modify that, but IMO I do not think you are approaching this the right way. Your interface logic seems flawed. If you have a list of vehicles that you choose from and once selected you get data related to that vehicle, then this data should be stored in a table. If you already have this table then the users should be adding a record to this table when the 'Other' option is selected.