Is there a systematic method for converting an even length FIR filter to odd length?
Standard even-length filters are somewhat inherent to 2-band real FIR orthogonal wavelets. And they are most often non symmetric.
If you really want odd length, or at least central symmetry, most probably you will have to relieve one of the above assumptions. The simplest could be to use some biorthogonal wavelets. The classical JPEG2000 9/7 pair is probably close enough to orthogonality for most purposes. Other filters can be tabulated, see for instance: Wavelet Biorthogonal 2.8 (bior2.8).
Then, there is a possibly to have complex filters, but honestly I am not familiar anymore with the literature.
You can also use $(2M+1)$-wavelet filter-banks, to quit the dyadic case. There are a couple of $3$-band wavelet designs with odd-length filters.