You could add another field to your data card called "Unit of Measure Length" and place it next to the Material Length property. This could be linked to a custom property in the model which can be pushed to the data card.
If you want to actually append it, that's not so easy without custom programming. You can create another field on your data card "MatlLengthFinal", then build an input formula for it in the text box properties to combine the Material Length field and the Unit of measure box". The problem with this is that the material legnth that is linked to the model property will not update input formula on the data card unless you type in one of the fields.