The Access Capture integration uses a 'Broker Service' along with a 'Mapping' configuration which sends the output data from the Invoice into the 'mapped' fields within Dimensions.
In order to set the Approver 1 with a specific UserID you must be:
Using Dimensions only for Invoice Register Approver
Have the correct Invoice Register settings applicable for either PL Approval or POP Approval within Dimensions Invoice Register settings
If you are using aCloud Approve or FocalPoint for PIR approval then it may not be possible to set a specific Approver 1 field as the setup does not allow for this.
In which case, if you would like to discuss your options you may require additional consultancy work to customise the integration, within the limitations of the system. In this case you would be advised to contact your Account Manager to discuss this further.