You can make a BED12 file extending your command above. Problem is the fields downstream are fixed values derived from the BED6. What are you planning to use this BED12 file for?
I used once the gtf2bed.pl program from the ea-utils. As far as I remember, it creates for each transcript a line in the resulting bed file. Thereby, it describes the transcripts' exon-usage, while loosing the transcript-gene association.
On the other hand, a GTF file implements the gene - transcript - exon relationships in an hierarchical way. This leads to a lot of redundant information.
You can make a BED12 file extending your command above. Problem is the fields downstream are fixed values derived from the BED6. What are you planning to use this BED12 file for?