Regulate 9VDC@7A PSU output down to 3 to 7.5 vdc at a 1Hz sine wave rate
The easiest way to do this would be with a microcontroller. You can easily create a lookup table that has whatever waveform you like (such as your offset sinusoidal waveform). Just output it to a DAC periodically, 'rinse and repeat'. 

For example, if you output a new voltage every millisecond you'd need a 1000 element table (or 250 if you use the symmetry of the sine wave). 

Vo = \$\text 5.25 + 2.25 V \sin(2\pi t) \$

The maximum rate of change is the derivative of voltage with respect to time at \$n2\pi\$, which is 14mV/ms, so that would be the maximum step between DAC updates, which is about 8-bit accuracy. 

One fairly painless approach would be to use a MCP4725 12-bit DAC chip (inexpensive boards are available) with a small Arduino. Total cost &lt; $10 plus the output amplifer- for which something like a $22 OPA541 (possibly overkill, but for a one-off perfectly justifiable) should work. Set it up for appropriate gain and you're all set.