Are you stating you wish to "mix" in the 7.83 Hz sine wave with the existing voice track? (An old-school term would be the 'sound-on-sound' process). If that is the case: Selectand Copy the sine wave data. Put the cursor at the head of the voice track (or wherever you want the tone to start), go to "Edit> Paste Special", and choose "Mix" . The mix window will have two volume faders, one for the 'Source' (copied sine wave), the other for 'Destination' (existing voice track) file. Choose OK to execute. FWIW, there are two shortcuts to 'Mix' copied data, as well; Right-click the cursor at the desired position on the timeline and choose 'Mix'.. or.. Ctrl+M on your keyboard. PS: You want both the voice and sine wave data the same sample rate and bit depth.
btw, welcome to the Magix Sound Forge users forum @Ram-Barg.