Initially I was unable to find a way and Oracle support reported that this indeed was not an option. However, after Oracle development has reviewed the enhancement request I filed they came up with a solution that is already in place. It turns out you can add additional target property fields however not from the GUI, you can only add them by using emcli.
The command to use to add additional target properties for a target in Oracle Enterprise Manager is to add_target_property. By using this command you can extend the number of fields per target type.
For example, if you wanted to extend all targets of the type "oracle_database" with a new target property to store the name of the system owner you could do so be executing the below command;
emcli add_target_property -target_type="oracle_database" -property="Owner Name"
In case you wanted to add the owner property to ALL targets you could do so by executing the below command:
emcli add_target_property -target_type="*" -property="Owner"
having this option will enable you to extend the options you have to store information on a target level in Oracle Enterprise Manager.