Building variable range incrementally using OpenSolver API

gistfile1.vb

Sub Test()
    Set TestSheet = Sheets("Test")
    OpenSolver.ResetModel TestSheet
    AddVariables Range("A2"), TestSheet
    AddVariables Range("B3"), TestSheet
End Sub

Sub AddVariables(VariableRange As Range, Sheet As Worksheet)
    Dim CurrentVariables As Range
    Set CurrentVariables = OpenSolver.GetDecisionVariablesWithDefault(Sheet)
    
    Dim NewVariables As Range
    If CurrentVariables Is Nothing Then
        Set NewVariables = VariableRange
    Else
        Set NewVariables = Union(CurrentVariables, VariableRange)
    End If

    OpenSolver.SetDecisionVariables NewVariables, Sheet
End Sub