Change current using PWM, for specific range outside PWM boundaries
According to the datasheet, these devices can be daisy chained.

So if you add another MCP41100 (or use a dual, MCP42100 for 100k devices) in series, you will add one effective bit, for 9 bit (512 step) resolution.

Note that the accuracy of these devices is very limited; I would probably put 4 in series for 10 bit resolution if I wanted to achieve decent precision.

There are quad devices available, incidentally.