How to safely merge two slices on harddisk?

[Rob]

Hi,

I have a hard disk, on which I would like to merge two slices
into one single slice. The disk slices are as follows:

/dev/ad1s1a    98M    43M    47M    48%    /home/userB
/dev/ad1s1d    64G    45G    14G    77%    /home/userA
/dev/ad1s1e   3.0G   2.5G   282M    90%    /home/userC
/dev/ad1s1f   3.0G   1.0G   1.7G    37%    /usr/ports
/dev/ad1s1g   3.0G   268M   2.5G    10%    /mnt
/dev/ad1s1h   295M   295M -23.5M   109%    /diskless_swap

I want to merge /ad1s1f and /ad1s1g into one 6Gb slice.

The merging should NOT destroy anything in the slices before
(ad1s1a, d, and e), but destroying the data in the one afer
(ad1s1h) is no problem.

Is there a way to do this? What is the safest one?
(without having to backup the whole disk).

Thanks,
Rob.