Acumatica add Virtual field to Data Grid from different table - acumatica

Question

I would like to add a virtual data field containing a custom db field UsrSubID in DB Table RQRequest.
I am trying to add the virtual field into a column in data grid on EPApproval screen. Below is my code on attend to use PXDBScalar to get the value for UsrSubID.
The relation between the two DAC is RQRequest.noteID = EPApproval.refNoteID
I get error:
error: The type name 'usrSubID' does not exist in the type
'PX.Objects.RQ.RQRequest' in file: Code#EPOwned(22)
Below is my code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using PX.Data;
using PX.Objects.CM;
using PX.TM;
using PX.Objects.CR;
using PX.Objects;
using PX.Objects.EP;
namespace PX.Objects.EP{
public class EPApprovalExt: PXCacheExtension<PX.Objects.EP.EPApproval>{
#region UsrCustomField
[PXString]
[PXUIField(DisplayName="Sub Account")]
[PXDBScalar(typeof(Search<RQ.RQRequest.usrSubID,Where<RQ.RQRequest.noteID,Equal<Current<EP.EPApproval.refNoteID>>>>))]
public virtual string UsrCustomField{get;set;}
public abstract class usrCustomField : IBqlField{}
#endregion
}
}

Answer 1

I will delete this answer after I help Fillip because I can't comment in his questions.
Fillip if it doesn't exists in DAC you can not get it. So why not declare it in DAC?
If that is default DAC of Acumatica, you can use your package and choose Add Field in your package, or you can create you own.
I think key work Graph Extension can help you with this.
Remeber, your DAC must have UsrSubID Field before you use it to another DAC or Graph. See HB_Acumatica comment