Can I add more 18650 cells to this led lantern but keep the same USB charging input, are there off the shelf PCBs available to do this ?

OEM cell is the blue one, pink cells are salvaged from another device.

 

You'd likely need a different BMS (Battery Management System) for multiple batteries. Simply paralleling a bunch of cells will not work out well if each battery is not properly managed. One battery will hog current leaving the others to not fully charge. The imbalance could result in a catastrophic fire.

 

One alternative that will be less complex, and can work if you have groups of cells that are similar to the original group, is to have a selector switch to chose which group is in use at a given time. The downside is that only one group would be charged at a time. A secondary benefit is that the groups of cells will not need to be matched at all, but only to have similar specifications.

And then, addressing Tony's line: find solutions to the problem, and then there are those who only find problems with the solution. Problems are sometimes not obvious, Part of my career was finding the secondary, unintended, consequences of various proposed actions proposed as solutions. secondary and even tertiary consequences are not always obvious, but in many cases they are not so happy.
an example: The solution to the problem of being broke is to borrow money. The consequence is having to pay it back eventually. The solution to paying back is to default on the payback. The consequences are even more unpleasant.

 

I'm expecting a whole new charging PCB with balance charging,

You'd likely need a different BMS (Battery Management System) for multiple batteries. Simply paralleling a bunch of cells will not work out well if each battery is not properly managed. One battery will hog current leaving the others to not fully charge. The imbalance could result in a catastrophic fire.

Is there an off the shelf BMS for 4 cells with USB input ?

 

OK, it seems that the suggestion of switching between groups of cells is not even worthy of comment. Way too easy, although the extension of the charging time will certainly be inconvenient.

But now a second issue: What condition are those other cells in?? If they are used, what is their current amp-hour capability?? Will the results actually be worth the effort?? Questions only the TS can answer.