Adding mass to a tonearm

Adding weights to the headshell DOES increase the effective mass of the arm. Not just because of the weight itself, but also because to then balance the arm, the counterweight on the back end will need to be moved further from the arm's pivot, which also increases the arms effective moving mass.

it doesn't really alter the mass of the tonearm that much because of the pivot

Even if you proceed with your experiment, you might just find that you don't have enough counterweight on the back of the arm. Headshell weights are usually for use with extremely light cartridges, but that isn't the case with 103R. 

I'd suggest trying the resonance test tracks on the HIFi News test record before and after any experiment. 

Also, a potential can of worms, but if you are planning this whole concept because of vinylengine calculator results "are you factoring in  the tendency for Asian, espec. Japanese, carts to establish their µm/mN compliance figure at 100Hz rather than the 10Hz typically used by the various on-line calcuators and formulas. Unfortunately many Asian manufacturers rarely publish that info. Euro carts generally use 10Hz.

I also understand, but am not an expert here, that there is no direct translation or conversion formula. The general rule I've read is to multiple the 100Hz compliance number by somewhere between 1.5 and 2.0, then use that result as part of the resonance frequency assessment. I use 1.75. Vinyl Engine gives the green light to resonance frequencies in the 8Hz to 11Hz range.". Cheers,
Spencer

As someone else wrote, adding mass to the headshell most certainly DOES increase the effective mass of the tonearm, by a factor roughly equal to the added mass, in grams. (As you move down the tonearm toward the pivot, the effect of adding mass at any point on effective mass lessens proportionately.)  Adding mass to the headshell will also cause you to need to move the counter-weight further back away from the pivot, in order to counter-balance the added mass and achieve the same VTF.  Doing that ALSO will increase the effective mass of the tonearm, by a factor equal to the square of the change in distance from the pivot to the center of mass of the CW, times the mass of the CW.  

From what I have been able to learn without owning a DL103 or 103R, there is almost no limit to the effective mass that those cartridges might "like".  One of my friends uses a home-made tonearm with his DL103 which appears to have an eff mass of nearly 50g! (Probably I am exaggerating, but it's mass-ive.)  I would suggest just experimenting with added mass at the headshell (and correspondingly re-balancing your tonearm) until you reach an effective mass that seems to be optimal for your cartridge. Then use a test LP to guesstimate the resonant frequency.