Solved! Go to Solution.
You need to use a formula tool to do this (this allows you to create fields).
And then you can use the statement...
[MyField]+"-01"
This will only work however, if the existing field you want to append to (which I have titled MyField), is already a string, otherwise you may have to use the statement...
TOSTRING([MyField])+"-01"
Or use a select tool prior to the formula to change the datatype.
Ben