How to use a parametric value within PULSE voltage source in LTSpice?
What @jonk said is true, but you can "cheat" a bit by using a parametric divisor, something like this:

PULSE(-15 15 0 {0.1u/x} {0.1u/x} {0.4u/x} {1u/x})
.param x=1
.step param x list 1 10 1000


Alternately, you can make it a bit more readable by making the rest of the values parametric:

PULSE(-15 15 {td} {tr} {tf} {Ton} {T})
.param T=1u/x Ton=0.4*T tr=0.1*T tf=tr td=0 x=1


This is just one way of doing it.