Yes, here's da script:
_xpos = _this select 0
_ypos = _this select 1
_letters = ["Aa","Ab","Ac","Ad","Ae","Af","Ag","Ah","Ai","Aj", "Ba","Bb","Bc","Bd","Be","Bf","Bg","Bh","Bi","Bj","Ca","Cb","Cc","Cd","Ce","Cf","Cg","Ch","Ci","Cj", "Da","Db","Dc","Dd","De","Df","Dg","Dh","Di","Dj", "Ea","Eb","Ec","Ed","Ee","Ef","Eg","Eh","Ei","Ej", "Fa","Fb","Fc","Fd","Fe","Ff","Fg","Fh","Fi","Fj", "Ga","Gb","Gc","Gd","Ge","Gf","Gg","Gh","Gi","Gj","Ha","Hb","Hc","Hd","He","Hf","Hg","Hh","Hi","Hj", "Ia","Ib","Ic","Id","Ie","If","Ig","Ih","Ii","Ij", "Ja","Jb","Jc","Jd","Je","Jf","Jg","Jh","Ji","Jj"]
_numbers = ["99","98","97","96","95","94","93","92","91","90", "89","88","87","86","85","84","83","82","81","80", "79","78","77","76","75","74","73","72","71","70", "69","68","67","66","65","64","63","62","61","60", "59","58","57","56","55","54","53","52","51","50", "49","48","47","46","45","44","43","42","41","40", "39","38","37","36","35","34","33","32","31","30", "29","28","27","26","25","24","23","22","21","20", "19","18","17","16","15","14","13","12","11", "10","09","08","07","06","05","04","03","02","01","00"]
?_xpos<0 or _xpos>12800:_xcoor="??";goto "Ycoor"
_xcoor = _letters select ((_xpos-64)/128)
#Ycoor
?_ypos<0 or _ypos>12800:_ycoor="??";goto "ShowXYcoor"
_ycoor = _numbers select ((_ypos-64)/128)
#ShowXYcoor
hint format ["%1%2", _xcoor, _ycoor]
The marker thing isn't possible I think. Markers cannot be created , only made visible or invisible. See the command reference