Can I charge a 42V SLA battery pack using a 48V SLA charger?
It is very likely that you will boil the batteries and destroy all of them.

12v batteries are designed to float charge to around 13.8v, so for a nominal 48 pack this will be 55.2v. Considering that a 12v battery has 6 2v cells, each cell is only rated to 2.3v on float charge. In this setup there would be 21 cells in a a 42v pack instead of 24 in a 48v pack. This means each cell would be charged to 2.63v and start boiling off all the electrolyte. 

It may be possible to find a charger which would allow you to adjust the float voltage but its not something that is likely to be listed on the specs page for the charger.