While one of the answers to the other question addresses this, it's not the chosen answer (which is wrong), and it doesn't really complete the reasoning, so I'm going to answer here.
The reason for adding whole peppercorns to soup and other long-cooking dishes has to do with flavor release. Piperine, the main flavor chemical in black pepper, is highly volatile, and does not dissolve in water. This is why, for most dishes, you add any black pepper near the end of cooking or even when plating the dish. If you add ground pepper any earlier, most of the piperine will be released by the ground pepper grains and cook out before you eat the dish.
For soups and other dishes where you want the piperine to be released slowly, and give it a chance to penetrate the ingredients (such as the chicken in the chicken pho) before cooking off, you want a "slow-release piperine". Fortunately you have one in the form of whole peppercorns.
You could definitely skip the whole peppercorns in pho, and replace them with ground pepper towards the end of cooking. However, you'd have to figure out several things:
- how much ground pepper to replace the more subtle whole peppercorn flavor
- when to add that pepper so that it wasn't either overwhelmingly peppery or cooked off and bland
And, even so, it wouldn't taste exactly like it would if you were following the recipe.
So, to summarize: no, pepper doesn't dissolve in water like salt, and, no, it would not be easy to modify the recipe.
My recommendation: find a cheaper place to buy peppercorns, like a discount food store or an Indian market.