I'm assuming you mean in an automated fashion (otherwise it's very simple XD). To do so, you have to use an SQL item_db2 (or have it imported, at the very least). You can then have a script pull all the items from it.
Here's an example script - simply send a whisper to "syncshop" on an admin account to refresh your shop with any changes in item_db2.
prontera,150,150,0 shop Example Shop 83,512:-1
- script syncshop -1,{
OnWhisperGlobal:
if (getgmlevel() < 99) end;
dispbottom "Syncing your shop, please wait.";
detachrid;
OnInit:
npcshopitem "Example Shop",512,15;
npcshopdelitem "Example Shop",512;
set .@count, query_sql("SELECT `id`,`price_buy` FROM `item_db2`",.@id,.@price);
for(set .@i,0; .@i<.@count; set .@i,.@i+1)
npcshopadditem "Example Shop",.@id[.@i],.@price[.@i];
end;
}