Can I add PWM timers to a microcontroller externally?
Yes, external PWMs can be added.  

In fact, there are I/O expanders intended for LEDs where each output has its own PWM channel.  The expander is controller through I2C.

PCA9532, MAX7313, for example.  There is no shortage of LED drivers like that.  Those datasheets will give you some keywords to find more.

There are also ICs specifically for driving segmented LED displays and matrices.  I had some success with AS1115.