Sample And Hold Adder
Lets take a look at your schematic.

The voltage at the output of the summing amp is \$Vsum = frac{V1 + Vsample}{2}\$.

When your switch is engaged (closed), the output voltage is \$Vsample = Vsum\$.

If we substitute our first expression into this equation, we get:

\$Vsample = \frac{V1 + Vsample}{2}\$

\$Vsample = \frac{V1}{2} + \frac{Vsample}{2}\$

\$\frac{Vsample}{2} = \frac{V1}{2}\$

\$Vsample = V1\$

So when the switch is engaged, the circuit will tend to push \$Vsample\$ towards the value of \$V1\$. When the switch is off, the sample stays the same (assuming no current loss through the op amp), and the value of \$Vsum\$ should be irrelevant.

Now that you have a proper sampler, Use a third op amp to add the DC value to the sine wave.