[GEM library] get a mappability wig track with fixed span

[pinkypeggy]

I am work with some selected region from hg18.
Now I want to generate the mappability score for 100mer in my selected regions.
I've done gem-indexer and gem mappablity. Now I want to convert the mappability to a wig file with fix span=100 foe every selected region.
But after gem-2-wig, I have a wig file like this
variableStep chrom=hg18_chr1_2965603_2985603_+ span=19901
1 1
variableStep chrom=hg18_chr1_2965603_2985603_+ span=99
19902 0
variableStep chrom=hg18_chr1_27511222_27531222_+ span=940
1 1
variableStep chrom=hg18_chr1_27511222_27531222_+ span=2
941 0.5

How can I fix the span=100 ?
Any suggestions are appreciated!