Two ICs actively driving same line
From the additional information provided by the original questioner it is clear that there will be a necessity to cut the trace between the existing hall sensor and the ApplicationUC. Two signals from the MyUC will be required, one of them being the signal to inject and one to control an added analogue MUX chip that gets added in to the circuitry roughly as follows:



Select an analogue MUX that can operate on the 3.3V that you said is your VDD. Make sure to be sure that the MUX can pass signals all the way from GND to the VDD range. The select digital output from MyUC will determine which entity can drive the analogue input at the AppUC. 

As indicated this is an rough idea diagram. There are some other things to work out here. One of those will be how low the ON switch resistance of the MUX has to be. Another thing will be to determine if the D/A converter on the MyUC has a low enough source impedance to drive the load at the AppUC. If the load impedance is fairly high (in the multi K ohm range) then it may be possible to use a low cost CMOS '4066 type chip for the MUX. If lower ON switch resistance is needed and/or smaller packages then take a look at the many analogue MUX parts from vendors such as Maxim, TI and Analog Devices.