In numbers or pages for ios, Is it possible to force a trend line to start at (0,0)?
The trendlines are based on the data and can't be forced thru any points. What you could do is have the app print out the equation for the trendline and plugin 0 to see what new trendline looks like. You can then adjust the coefficients so that you have a new trendline thru 0.0. This may not be a good representation for the trendline. 

Another possibility is to add several pairs of 0.0,0.0 to the row and column you are plotting. Start with just 2 or 3 pairs and see how that affects the trendline. Depending on your trendline, you may need more pairs. this will weight that part of trendline more than other parts and will cause a shift in the line.

It should be noted that the trendline you enforce will probably not represent the data very well. If the value at 0.0 is actually 245, while the other values you have are in the range of 200-250, for example, forcing it to zero at zero is a bad idea. Adding data points is also a bad idea because you inputting data that doesn't exist grossly changes your data set.

The trendline is a representation of the data in a relatively simple manner. Changing it makes it no longer representative of the data. If you suspect your data at 0.0 should be 0.0, then this is probably a method to determine that.