There may be some options on how to handle this situation but you have left out some pretty important information. Missing is how high the existing 4x6 supports are already.
It the posts are only 6 inches high then it would be pretty hard to add 30 inches on top of them. On the other hand if they are 4 feet high then the possibilities increase.
Given enough length to the existing posts it may be feasible to place vertical 2x6 planks on each side of the existing post plus the added 30 inch length. These planks can be bolted on by drilling through the whole affair. A stub of 4x6 that is 30" long can be set between the top of the 2x6's and bolted in place as well.
The added length of the posts may warrant adding some triangulation braces for support. A horizontal member added between posts at the height of the split, in conjunction with the angle braces, would help to prevent racking at that location.