The power source is a CC/CV charger, meant to charge an 8S (max 33.6V, min 26.4V) lithium battery. Let us assume that it is limited to 3A.

I need to to be able to also charge a 10S battery (max 42V, min 33V)

My initial thought was to add a boost converter to the battery circuity for the 10S battery. This would stay with the battery by the BMS.

Is there a way to do this? I'm slightly confused as to how boost converters function when their input current is limited. Ideally, once the boost converters output reaches 42V and the battery is fully charged the circuit will not draw any (or very little) current from the charger so that it can still operate its charge indication LED on the charger.

It's important to remember that the boost converter can't supply a constant voltage. If it tried to output a constant 42V the batteries would draw a lot of current trying to equalize charge. So the boost converter has to operate relative to its power input.

I'm not interested in modules I can purchase for this, I need to integrate this into a PCB I'm designing. If you can't tell, I'm much more experienced in digital electronics, not analog.

 

Look up how Lithium batteries charge at www.batteryuniversity.com . The battery is far from fully charged when its current-limited voltage reaches 4.2V per cell because it is still charging.
Also, a multicell lithium battery is balanced-charged which senses and adjusts the voltage of each cell and preventing a cell or more from being overcharged then exploding.
A good lithium charger senses the uncharged battery voltage and if it is less than 3V per cell then it tries charging with a low current. If the voltage does not normally increase then it shuts down the charging with a warning, to prevent an explosion.

 

I'm aware of how a lithium battery operates. Both the 8S and 10S packs I'm using have integrated BMS's so that they can be charged with a single CCCV source.

And yes, once the charger voltage reaches battery voltage it continues to charge. The battery is only full once the charge voltage reaches the maximum charger voltage and current drops to 0.

Hobbyist lithium chargers are a ripoff. They only cost so much because they can balance charge many different sizes of batteries. It's much easier, and user-friendly, to have an integrated cell balancer inside the battery and only charge it through a single 2 prong port.