Is there a good way to define them yourself?
Like others have mentioned, Excel can not work well with Units like a
dedicated Math program like Mathematica. Excel can not keep track of
numeric, and a separate symbolic units.
One poor-mans way to do something like this would be to use Range Named
constants. It depends on what you are doing of course. If you wanted to
work in hours, here is a small example. It is not the best idea, but it may
give you some ideas for your situation.
Sub Demo()
ActiveWorkbook.Names.Add Name:="miles", RefersTo:="=1"
ActiveWorkbook.Names.Add Name:="hour", RefersTo:="=1"
'1 Hour = 60 Minutes
ActiveWorkbook.Names.Add Name:="minutes", RefersTo:="=hour / 60"
[A1].Formula = "=60*miles / hour"
[B1].Formula = "=2*hour"
' Or...
[B1].Formula = "=120*minutes"
[C1].Formula = "=A1*B1"
End Sub