Is it possible to default certain values for particular fields?

Yes it is possible to default values for certain fields where a parameter id is present.
Step 1 Go to the input field to which you want to make defaults.

Step 2 Press F1, then click technical info push button. This would open a window that displays the  corresponding parameter id (if one has been allocated to the field) in the field data section.

Step 3 Enter this parameter id using the following path on SAP Easy access screen System à User profile à Own data.

Step 4 Click on parameter tab. Enter the parameter id code and enter the value you want as default. Save the usersettings.