The behavioural elements include, besides the voltage and current, resistor, inductor, and capacitor. If you ran a quick search through the manual you would have found out quicker than it would have taken you to ask here: LTspice > Circuit Elements > C, bottom. Simply place a capacitor and add
Q=<...> as the value. If the formula includes
x, that signifies a derivative of the current, so your expression needs to be integrated first -- unless you don't need
x, it can be done, for example external voltage, in your case.
So, for example, if you want your capacitance to vary according to \$\sin x\$, then you have to integrate that first, which gives you \$-cos x\$:
See the expression is
x is the derivative of the current. The driving voltage is a unity ramp, which means the current through the capacitor is directly its value (
I(C1)). The behavioural voltage is for confirmation,
V(test), which shows the same sine. Here, for reasons of clarity, I avoided dividing by 2\$\pi\$, so that the current has a different amplitude; had I not, the current and the voltage would have overlapped and it would not have been clear.
Being a behavioural element, it is dependent on time and its effects, in LTspice, in addition to the derivative of the current you risk getting a lot of noise, so be careful what you wish for.