"increase the gears".

What front chainwheel and rear cog teeth combinations (numbers of teeth) do you have at present?

What exactly is lacking in the selection of 18 gears you have currently, that you need to "increase"?

Are you struggling on hills at the low end, "spinning out" at the high end or what? Adding more gears, many of which will be redundant, will not make you go faster, better conditioning will.