Is there an equivalent to the the HS.con function?

Yes, there is, it is called setCell and it is a method of the databuffer class. It doesn't use a string as input, so it is a little different to use

resultDataBuffer.SetCell(si As SessionInfo, cell as DataCell, accumulateIfCellAlreadyEsists as boolean)

or

resultDataBuffer.SetCell(si As SessionInfo, cell as DataBufferCell, accumulateIfCellAlreadyEsists as boolean) 

The Onestream help is giving a similar example like this one for the whole process.

'Copy all "U2#Input:U3#Input" numbers for this dataUnit to the UD2 none Members and the UD3 member with the name "Target Member Name".
Dim destinationInfo As ExpressionDestinationInfo = api.Data.GetExpressionDestinationInfo("")
Dim sourceDataBuffer As DataBuffer = api.Data.GetDataBuffer(DataApiScriptMethodType.Calculate, "U2#Input:U3#Input", destinationInfo)
If Not sourceDataBuffer Is Nothing Then
    Dim resultDataBuffer As DataBuffer = New DataBuffer()
    For Each cell As DataBufferCell In sourceDataBuffer.DataBufferCells.Values
       If (Not cell.CellStatus.IsNoData) Then
          cell.DataBufferCellPk.UD2Id = DimConstants.None
          cell.DataBufferCellPk.UD3Id = api.Members.GetMemberId(dimtypeid.UD3, "Target Member Name")
          resultDataBuffer.SetCell(si, cell)
       End If
    Next
    api.Data.SetDataBuffer(resultDataBuffer, destinationInfo, True)
End If