Is there any simple solution to adding a thousands separator to a string that is suffixed with letters?

for example, I get data in as such:

```
123456 km

```

And I would like to turn that into:

```
123,456 km

```

is there a simple solution short of stripping the data of the suffix, doing `String.Format("0:n0")`, then re-adding the suffix back in?

Edit: Suffix not Prefix.