Instead of bigWigToBedGraph, you might use bigWigToWig, instead, as bedgraph is BED-like but isn't exactly BED, which may affect use of UCSC's liftover tool. In any case, both binaries are available from the UCSC executables download page.

Once you have the file in WIG format, you can convert it to a UCSC BED file with the BEDOPS wig2bed conversion utility.

Alternatively, you could take the bedgraph output and convert it to BED with an added GNU awk step:

$ awk '{ \
    if ($1 ~ /^chr/) { \
        print $1"\t"$2"\t"$3"\tid-"NR"\t"$4; \
    } \
}' foo.bedgraph \
> foo.bed

You should be able to safely run it through liftover at that point.

bigWigToBedGraph might be what you're looking for.

bedgraph format