Can you alter sand with topsoil composite or mulch?

The mixture that you ideally want will depend largely on what you want to grow and the amount of rain you receive. For example, if you want garden soil for planting veggies, the best soil is typically a loamy soil. You can make a sandy-loam soil soil mix as follows: 1/3 part sand + 1/3 part silt (use compost) + 1/3 part clay (use topsoil). Note: If you live in a droughty area, you may want a bit more silt & clay in the mix. If you live in a relatively moist area, you may want to reduce your clay part to maybe 1/4 of the mix.

If you do the research, you will typically see recommendations to NOT add more than 2" of finished compost to your soil per growing season, but I have mixed up to 50% finish compost with my heavy clay soil and not had any problems. In fact, my best potting soil mixture for growing citrus trees is about 2/3 finished compost + 1/3 clay (my topsoil). I have experimented with adding sand to my mixtures, however it tends to cause the water permeation rate to slow down more than I prefer, so I usually don't add any sand to my soil mixes.

You can mix mulch into your soil, but I wouldn't recommend it unless it is very well decomposed. Bark & wood pieces definitely add nutrients to the soil, however they can also use up some of the available nitrogen in the soil during the decomposition process. That means less nitrogen is available for your plants to grow. If you live in a climate that has high evaporation & your soil tends to dry quickly, use the mulch on top of the soil to reduce evaporation and to add trickle-down nutrients as the mulch decomposes. The mulch also helps to suppress weed growth--which is plus for most gardeners. :)