>declare @FieldsToAdd table (FieldName sysname, FieldExtra varchar(800)) -- assuming you only adding fields and never changing type, size, etc.
>declare @Script nvarchar(max)
>select @Script = ''
>select @Scipt = @Script + char(13) + 
> 'ADD ' + quotename(FieldName) + ' ' + FieldExtra from @Fields F
>where not exists (select 1 from Information_Schema.Columns where Table_Name = @TableName and Column_Name = F.FieldName)
>
>set @Script = 'ALTER TABLE ' + quotename(@TableName) + @Script
>
>print @Script -- testing
>
>