TP4056 to charge xS1P li-po li-ion
it's not clear to me if it's possible to only charge a battery pack with several cells in series.

It is clearly answered in the datasheet: 

DESCRIPTION
The TP4056 is a complete constant-current/constant-voltage linear charger for single cell lithium-ion batteries.

At the end of page 3 there's a schematic which will be very similar to what is on most TP4056 modules:



Note how the - terminal of the battery is also grounded. On the module you can also easily see (and measure) that IN- / GND and B- are shorted. So indeed, the TP4056 is not suited for balanced charging of series connected cells.

If you need to charge series connected cells then you need a different IC.