Encontrei a solução:

Function FillSpace(nPar As Variant, nOccurs As Integer, L_R As Boolean) As String

Dim nSize As Integer
Dim nParticle As String

Let nSize = Len(Trim(nPar))

' Informa se será preenchido na frente (LEFT) ou atrás (RIGHT).
If L_R Then
Let nParticle = Space(nOccurs - nSize) & Trim(nPar)
Else
Let nParticle = Trim(nPar) & Space(nOccurs - nSize)
End If

Let FillSpace = nParticle

End Function

'FillZeros((valorrs), 15, True)