Help for the Numeric Integrator.vi shows the formulas. For the trapezoidal rule (the simplest one) the integral is the sum of partial sums which are given by: 1/2(x[i] + x[i + 1])*dt.
If you change dt to dt(i) = t[i+1] - t[i], I think this should give you a reasonable approximation. The more complex rules cannot use this approach. The trapezoidal rule simply sums the area under the trapezoids created by connecting the t[i], x[i], x[i+1], t[i+1] dots, so this change should not introduce any errors.
