I'm gonna assume you're not a coder by the fact that you asked this question. Luckily for you, adding brickmania levels is relatively easy.
First, you need to set up a build environment and learn how to create a Rockbox build:
http://www.rockbox.org/twiki/bin/view/Main/DevelopmentGuideYou're looking for this section in
apps/plugins/brickmania.c:
static unsigned char levels[29][8][10] = {
{ /* level1 */
{0x1,0x1,0x1,0x1,0x1,0x1,0x1,0x1,0x1,0x1},
{0x2,0x1,0x0,0x0,0x0,0x0,0x0,0x0,0x1,0x2},
{0x0,0x2,0x1,0x0,0x0,0x0,0x0,0x1,0x2,0x0},
{0x0,0x0,0x2,0x1,0x0,0x0,0x1,0x2,0x0,0x0},
{0x0,0x0,0x0,0x2,0x1,0x1,0x2,0x0,0x0,0x0},
{0x7,0x0,0x0,0x7,0x2,0x2,0x7,0x0,0x0,0x7},
{0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0},
{0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0}
},
...
};
Obviously each of those "0x0", "0x1", etc. numbers means a certain type of brick.
You need to increment that number 29 there every time you add a level.