reuse of partitions

I was wondering if there is option to reuse the partitions present by preseeding the Ubiquity?

I would like to use the first partition for /, second one partitioned for swap, third for /home and so on..

The below expert_recipe does partition the disk but erases the existing partitions. And method{ keep } also didn't help either.

ubiquity partman-auto/expert_recipe string \
      myboot-part :: \
              10240 1049 10240 ext4 \
                      $primary{ } $bootable{ } \
                      method{ format } format{ } \
                      use_filesystem{ } filesystem{ ext4 } \
                      mountpoint{ / } \
              . \
              16384 8172 16384 linux-swap \
                      $primary{ } method{ swap } \
              . \
              30720 10240 30720 ext4 \
                      $primary{ } method{ format } format{ } \
                      use_filesystem{ } filesystem{ ext4 } \
                      mountpoint{ /backup_fs } \
              . \
              10240 10240 10000000 ext4 \
                      $primary{ } method{ format } format{ } \
                      use_filesystem{ } filesystem{ ext4 } \
                      mountpoint{ /home } \
              .