Something like this perhaps?
g = 9.81; l0 = 0.2;
Manipulate[
sol = NDSolve[{x''[t] == (l0 + x[t])*(theta'[t])^2 - (k*x[t])/(m) +
g*Cos[theta[t]],
theta''[t] == (-2*theta'[t]*x'[t])/(l0 + x[t]) - (g*
Sin[theta[t]])/(l0 + x[t]), x[0] == 0.05, x'[0] == 0,
theta[0] == 0.35, theta'[0] == 0}, {x, theta}, {t, 0, 100}];
fx = x /. sol[[1, 1]];
ft = theta /. sol[[1, 2]];
pl1 = ParametricPlot[{t, fx[t]} /. sol, {t, 0, 40},
PlotRange -> {{0, 6}, {-1, 3}}];
pl2 = ParametricPlot[{t, ft[t]} /. sol, {t, 0, 40}];
Show[pl1, pl2],
{{m, .9}, 0, 2}, {{k, 10}, 0, 20}]
What do you mean by "approximate function to these graphs"?