Hemalurgists are unusual among the characters on this site in that they have the potential to develop additional powers beyond those they already have, however, this involves the death of a person with the power they are seeking to acquire. Hemalurgy is also an incredibly rare and prohibited practice, there has to be a good justification for your character to possess the knowledge to practice it.
A Hemalurgist app may never be created with more than four allomantic abilities and they must kill other Allomancers to acquire more. They can acquire them from other players characters, but only with the permission of said player. They can acquire powers from NPC Allomancers, but only with the express consent of a member of staff. A player may not kill their own characters to increase the powers of their Hemalurgist unless it is first cleared by a member of the staff.
As a Hemalurgist gains more powers, their abilities grow slightly weaker, so a Hemalurgist with only Pewter would be able to burn it more strongly than a Hemalurgist with Pewter and Tin. In addition, a Hemalurgist's abilities will never be as strong as those of a Misting, unless they also possess the ability to burn Duralumin.